The present invention relates to a precipitation preventing method for a silicone composition and more particularly to a method for preventing the precipitation or settling of fillers contained in a silicone composition, the caking of precipitated fillers, the loss of fluidity, etc.
Silicone compositions, or polyorganosiloxane compositions, are well known and have heretofore been used in various fields. As polyorganosiloxane compositions there are well known addition reaction type polyorganosiloxane compositions and condensation reaction type polyorganosiloxane compositions.
The former, addition reaction type polyorganosiloxane compositions, are of the type wherein a polyorganosiloxane having an alkenyl group and a polyorganosiloxane having a hydrogen atom bonded directly to a silicon atom are cured by using an addition reaction catalyst, e.g. a platinum compound.
The latter, condensation reaction type polyorganosiloxane compositions, are of the type wherein a polyorganosiloxane having a hydroxyl group and a hydrolyzable crosslinking agent are cured by using a tin compound for example.
Fillers are often added to these silicone compositions for imparting usuful properties thereto and for reducing the cost.
Various filler-containing silicone compositions are known. For example, the silicone potting agent used for the embedding of electric and electronic parts possesses superior characteristics after curing such as high heat resistance, cold resistance, water resistance and moisture resistance, as as well as excellent shock absorbing property, electrical characteristics and heat conductivity, so is applied to various uses. Such conventional electronic parts as power transistors, diodes, coils and various printer heads generate heat during use, which may lead to deterioration of their characteristics or damage, so various silicones for heat radiation are used. Also, RTV silicone is used for sealing the flange surfaces in automobiles, construction machines, industrial robots, etc. from fluid.
The following are mentioned as examples of fillers highly effective for both addition reaction type and condensation reaction type (one-part, two-part) silicone compositions. Examples of heat-conductive fillers include such metal oxides as zinc oxide, aluminum oxide, feric oxide, titanium dioxide, and zirconium oxide; such metals as copper, copper alloy, aluminum, and iron. Further, in uses in which corrosion should be avoided, there are known highly heat-conductive ceramics such as graphite and silicon carbide.
Examples of other fillers which are used frequently include fumed silica (AEROJIL), precipitated silica, which enhances cured physical properties, tensile strength and tear strength, quartz powder, diatomaceous earth, calcium carbide, glass fiber, calcined clay, mica powder, potassium titanate, and chromium oxide. These may be used each alone or in combination of two or more.
Usually, silicone compositions are available commercially in one- or two-part form containing such fillers, catalyst, etc. Electrical characteristics, shock absorbing property, heat radiating property, as well as resistance to heat, cold moisture and chemicals, of the silicone compositions are enhanced by the fillers. However, since the fillers are incorporated in the silicone stock solution usually in a large amount of 30 to 70 wt %, if a treatment is performed to keep low the viscosity of the silicone composition used, that is, if priority is given to the easiness of use on the user side, the fillers will be precipitated and so it is necessary to make mixing and agitation at every use of the composition. Further, if the composition is stored for a long period in a distribution channel, the precification will proceed and cause caking of the fillers, thus resulting in that it becomes difficult to effect mixing and agitation at the time of use. On the other hand, increasing the viscosity for suppressing the precipitation will lead to the loss of fluidity and of the easiness of use on the user side.